Chelated Ruthenium Catalysts for Z-Selective Olefin Metathesis

Abstract

We report the development of ruthenium-based metathesis catalysts with chelating N-heterocyclic carbene (NHC) ligands which catalyze highly Z-selective olefin metathesis. A very simple and convenient synthetic procedure of such a catalyst has been developed. An intramolecular C-H bond activation of the NHC ligand, which is promoted by anion ligand substitution, forms the appropriate chelate for stereo- controlled olefin metathesis.

Recent Advancements in Stereoselective Olefin Metathesis Using Ruthenium Catalysts

“…When attempts were made to generate 2, it was discovered that C-H activation of an ortho-methyl in N-2,4,6-trimethylphenyl (Mes) occurred to generate a cyclometalated catalyst, 3 ( Figure 2). Although cyclometalated ruthenium complexes have been reported as catalyst decomposition products [32,33], 3 performed the cross metathesis (CM) of allylbenzene and cis-1,4-diacetoxybutene in moderate conversion (58%) and the highest observed Z-selectivity for a ruthenium metathesis catalyst (at the time), 41% Z [34]. Following this result, it was proposed that a larger N-heterocyclic carbene (NHC) could enhance the steric demands of the catalyst, favoring an all syn-ruthenacycle, so one of the N-Mes groups was replaced with an N-adamantyl.…”

“…Following this result, it was proposed that a larger N-heterocyclic carbene (NHC) could enhance the steric demands of the catalyst, favoring an all syn-ruthenacycle, so one of the N-Mes groups was replaced with an N-adamantyl. Surprisingly, C-H activation occurred on the N-adamantyl group (4), which resulted in a more selective OM catalyst (88% Z in the CM of allylbenzene and cis-1,4-diacetoxy-2-butene) [34]. Computational and experimental studies were carried out to understand the Z-selectivity imparted by these cyclometalated catalysts [35,36].…”

“…The understanding developed from previous generations of ruthenium metathesis catalysts [45] was applied to the new cyclometalated catalysts. Studies using 4 commenced by probing the reactivity and selectivity with a CM system that included allylbenzene and cis-1,4-diacetoxy-2-butene ( Table 2, entry 1) [34]. The desired CM products were isolated in moderate yield, but high levels of Zselectivity were observed.…”

Title Pages

“…Several new ruthenium N-heterocyclic carbene complexes have been fashioned for specific aspects of olefin metathesis, the most notable being another offering of Grubbs, based on 1-adamantyl-3-mesitylimidazolidin-2-ylidene as ligand, specifically designed for efficient synthesis of challenging (Z)-olefins 25 and considered an improvement/alternative to recently reported molybdenum complexes for the same purpose 26 . An efficient cross-metathesis and ring-closing metathesis of ethyleneammonium salts (including primary amine salts) is also reported 27 , reaction with ethyleneamines generally being unsuccessful.…”

Stereoretentive Olefin Metathesis: An Avenue to Kinetic Selectivity

“…Later, Grubbs and coworkers introduced a highly efficient, Z-selective olefin metathesis catalyst that utilized a cyclometalated rutheniumcarbene species. [27][28][29][30][31][32][33][34][35][36] The aryl group on the N-heterocyclic carbene (NHC) forces all the substituents on the ruthenacycle syn ( Figure 1 (b)). Subsequent reports by the Jensen [37][38][39] and Hoveyda [40][41][42][43] groups also described Z-selective ruthenium catalysts.…”